So recently I noticed in task manager my CPU speed is showing as 0.78 GHz, my processor is a 2.5 GHz. This shows as 2.49GHz in safe mode. I'm curious what the issue could be as it does not change from 0.78 no matter what I do.

This is the result of the power management capabilities enabled in your processor. The processor clock should move up to higher speeds as the processor is asked to do more work. When you say "no matter what I do", what did this entail? Try doing something like forcing Windows Update (or equivalent if running Linux) to run and you should see the clock speed increase...

If you find that it never increases, it is possible that the power management configuration has locked it at this lower clock speed. This can be configured from within the Power Options applet in Control Panel. You can change to the High Performancepower plan if you want to lock it at the highest clock speed. Alternatively, click on the Change Plan Settings link for your current power plan and then click on Change Advanced Power Settings to get into the editor for individual power parameters. In the Processor Power Managementsection, you can set the Maximum Processor Stateparameter to a higher percentage (like 100%, where it should default) to increase the maximum allowed clock speed.

So "no matter what I do" entails anything whether thats playing games etc, the speed does not change no matter what, I'm on high performance and I even set balanced and power saver to 100% mininum and maximum and it still hasn't changed.

Ok, next thought is to check whether this is just an issue with the (Windows) display of the frequency (there are reports of this kind of issue on the web). Download and run the Intel Processor Frequency Id Utility. The download page is here: Drivers & Software. Both Windows and standalone (bootable) versions are available...

That program doesn't support the new systems, but the new version "Intel Processor Identification Utility" shows 0.80GHz.

The following is the ins and outs of the story so far:

I followed all the info supplied by the various manufacturers but mainly ASUS MB instructions and videos, the system fired up and works as expected with no issues except one very big issue that i cannot fathom, the CPU is fixed at 0.77GHz speed, that’s 22% of the standard 3.5GHz expected from the "Intel I5 6600K", this is on the "ASUS Z170 Deluxe" MB with "G.Skill Ripjaws V Black 16GB (2x8GB) DDR4 PC4-25600 3200MHz Dual Channel Kit ".I have checked the Windows 10 home OS is set on "performance" mode, the MB BIOS i updated to the newest S/W version "1801" before even building the rig following the official ASUS guide videos and hand book, i have tried running standard then with XMP enabled, then used the ASUS "Dual Intelligent Processors 5" program to automatically OC the system and it got to 4.7GHz (it said) and still actual speed is 0.77GHz (using CPUz and Task Manager to check). I have stressed the CPU using games, CPUz stress test and various other means but 0.77 is maxed out.I have completely reset everything, clean M.2 drive, clean install of Windows 10, BIOS returned to stock values, cooling not an issue, even if i turn ALL the fans OFF just using the water pump the hottest i can get the CPU is 36*C, MB 38*C, PCH 45*C and VRM 38*C. Power is also no issue with the RM850 watt from Corsair.I have checked the CPU with "Intel Processor Diag Tool 645.W.MP", everything was a Pass in green including the CPU.I can also see in the UEFI (BIOS) screen that CPU speed is listed as 800MHz even with no O/S installed (no H/D connected). This would rule out O/S intervention.I have contacted Asus support and they said with the testing that i have already done it would be doubtful to be S/W issue or a setting restricting the CPU speed. They adviced i run the system out of the case (of course on non-conductive surface) with only the basics connected and everything on its basic default setting, that made no difference. They also asked that i carefully check the CPU socket and CPU connections for damage/bent pins and contamination, these checked out fine, i even photographed the socket and CPU so could blow it up on a 24" screen, everything looks new.

I’m at a loss as to what is at fault, the speed limited to 22%, the BCLK at 100.2 x 8, the multiplier will not move of "X8" even though it has at best shown "x8 - 47" but still running at only “x8”, CPU core V at 0.813 (i have increased this to 1.225V but exactly the same 22% on CPU). I have also tweaked many other values a little but as there was no response from altering these i have put all back to standard rather than make matters worse. I have reset CMOS and locked the 3200MHz memory to 2133MHz (2x8GB).

I am not an expert but i have built and repaired many computers over the last 15 years so I’m not new to this work but this one has got me beat. I believe there is a H/W issue with either the CPU or the motherboard but i do not have access to (or the money to buy new) another 1151 type CPU or motherboard.